CD8+ cytolytic T cells serve a critical role in the immune response against many intracellular pathogens, particularly viruses, through the specific recognition and selective elimination of infected host cells. CD8+ T cells discriminate between infected and healthy cells on the basis of unique, pathogen-derived peptide epitopes presented by MHC class I molecules on the surface of infected cells. The importance of the MHC class I antigen presentation pathway in anti-viral immunity is underscored by the growing realization that many viruses encode molecules that disable class I antigen presentation. There is striking diversity in the class I inhibitor pathways utilized by different viral molecules. Recently, it was discovered that many gamma-herpesviruses and poxviruses encode homologous ubiquitin ligase molecules which these viruses employ for evasion of host immune responses through the ubiquitin-mediated destruction of MHC class I molecules and costimulatory molecules such as B7.2 and ICAM-1. Of the molecules characterized to date, mK3 of gamma-herpesvirus (HV) 68 is unique in that it catalyzes the addition of ubiquitin to the cytosolic tail of ER-resident MHC class I heavy chains, and this process requires the MHC class I peptide-loading complex (PLC) to facilitate the targeting of class I molecules for destruction by the mK3 protein. These new findings represent an excellent opportunity to unravel the molecular details of the mK3-mediated degradation in the context of class I assembly. In a broader sense, further characterization of mK3 will provide new insights into ubiquitin-mediated immune evasion pathways utilized by other viruses (such as Kaposi's sacrcoma-associated herpesvirus) and normal cellular processes involving ubiquitin, which have been increasingly implicated in the regulation of immune responses. To this end, the research proposed here will define: 1) the molecular details of the interaction between mK3 and the PLC, and define the basis for mK3 substrate selection. 2) The physiologic importance of the mK3 pathway during in virus infection. 3) The cellular ubiquitin pathway components exploited by mK3. 4) Potential immune modulatory functions of normal cellular homologs of mK3.